1. Field of the Invention
This invention generally relates to a multistage sprocket assembly for a bicycle. More specifically, the present invention relates to a sprocket assembly having at least one larger diameter sprocket and at least one smaller diameter sprocket with the teeth of at least the one larger diameter sprocket being dimensioned to allow for smoother shifting of the bicycle chain between the smaller sprocket and the larger sprocket.
2. Background Information
Bicycling is becoming an increasingly more popular form of recreation as well as a means of transportation. Moreover, bicycling has also become a very popular competitive sport for both amateurs and professionals. Whether the bicycle is used for recreation, transportation or competition, the bicycle industry is constantly improving the various components of the bicycle. The drive train of the bicycle has been redesigned over the past years. Specifically, manufacturers of bicycle components have been continually improving shifting performance of the various shifting components such as the shifter, the shift cable, the derailleur, the chain and the sprocket.
One particular component of the drive train that has been extensively redesigned is the sprocket assembly for the bicycle. More specifically, the bicycle sprocket assembly has been improved to provide smoother shifting.
Conventionally, a multistage sprocket assembly includes a smaller diameter sprocket and a larger diameter sprocket assembled such that: (1) the center point between a pair of adjacent teeth at the larger diameter sprocket and the center point between a pair of adjacent teeth at the smaller diameter sprocket are positioned on the tangent extending along the chain path; (2) a distance between the aforesaid center points is an integer multiple of the chain pitch; and (3) a first tooth of the larger diameter sprocket positioned behind the center point between the adjacent teeth at the larger diameter sprocket in the rotation direction for driving the bicycle is made to be easily engageable with the driving chain, thereby improving the speed change efficiency when the driving chain is shifted from the smaller diameter sprocket to the larger diameter sprocket.
The drive chain is a continuous loop that has a plurality of inner link plates and a plurality of outer link plates that are pivotally connected to each other by articulation pins and rollers. The space between the opposite surfaces of each pair of inner link plates is smaller than that between the opposite surfaces of each pair of outer link plates. In other words, each pair of the outer link plates is positioned outside the inner link plates and forms a space larger in width, while each pair of the inner link plates is positioned inside the outer link plates and form a space smaller in width.
The driving chain constructed as described above is biased by a derailleur toward either a larger diameter sprocket or a smaller diameter sprocket so as to be shifted thereto. Specifically, during a chain shifting process, the chain is shifted from one sprocket to the next adjacent sprocket by the rear derailleur moving the chain in an axial direction relative to the axis of rotation of the sprockets. By modifying the teeth of the large sprocket, the chain can execute smooth down shifting or up shifting motions. An up shift occurs when the chain is moved from a large sprocket to the next smaller sprocket. A down shift occurs when the chain is shifted from a small sprocket to the next larger sprocket.
Basically, when the sprocket assembly is rotated in a driving direction, the inner and outer link plates engage the teeth of one of the sprockets. In the case of a sprocket with an even number of teeth, the inner and outer link plates will always engage the same teeth. In the case of a sprocket with an odd number of teeth, the inner and outer link plates will alternately engage different teeth with each rotation of the sprocket assembly. Therefore, the teeth of the sprockets will alternately engage both the inner and outer link plates. The teeth of a sprocket with an odd number of teeth are typically designed to accommodate shifting with either the inner or outer link plates engaging the up shift teeth. Thus, the teeth of the sprocket must have one shift path for the inner link plates and another shift path for the outer link plates. If the chain is shifted on the wrong shift path, the rider will most like experience pedaling shock.
One example of an improved sprocket assembly is disclosed in U.S. Pat. No. 4,889,521 to Nagano. While the sprocket assembly disclosed in the Nagano patent operates very well in shifting from a smaller sprocket to a larger sprocket, no provision has been made for shifting from a large sprocket to a small sprocket. Moreover, Shimano introduced the Interactive Glide (IG) sprocket with the basic design of Shimano""s HyperGlide (HG) sprocket plus uses new ramps and teeth configurations to control up shifts.
One example of a sprocket incorporating up shifting and downshifting paths is disclosed in U.S. Pat. No. 6,045,472 to Sung et al. The Sung et al. patent shows a sprocket designed to have two up shifting paths adjacent to each other. The Interactive Glide (IG) sprocket also has two up shift paths because of a combination problem of a sprocket with an even number of teeth and the outer link plates of the chain. In order to solve this problem, two up shifting paths were arranged adjacent so that one of the up shifting paths contributed for up shifting depending on the combination of the inner and outer links of the chain contacting the teeth of the sprocket.
More specifically, referring to FIG. 3 of the Sung et al. patent, when the outer link plate of the chain meshes with tooth 13, the inner link plate of the chain is guided by 4B. However, when the outer link plate of the chain meshes with tooth 14, the inner link plate of the chain is guided by 4C. Accordingly, design of 4B and 4C are different. The radial position of 4C is higher than 4B to take up slack of the chain from the second up shift escape point to a second up shift engagement point. Consequently, up shifting performance in the first up shift path and the adjacent second up shift path is different. One up shift path is superior to the other up shift path and such superior up shifting is so smooth that pedaling shock can be prevented. However, the other up-shift path is not so smooth and pedaling shock can occur.
In the course of up shifting the chain from the large sprocket to the small sprocket, the chain may ride on a tooth crest of either the small or large sprocket thus interfering with the chain shift without proper phase arrangement between the teeth of the large and small sprockets. If this happens, the rider will most like experience pedaling shock.
In view of the above, there exists a need for an improved sprocket assembly assuring smooth and reliable chain shift action from the large sprocket to the small sprocket. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
One object of the present invention is to provide a sprocket that is designed to provide a superior up shifting path by modifying the sprocket teeth to assure smooth and reliable chain shift action from the large sprocket to the small sprocket.
The rider can enjoy smooth up shifting without pedal shock during up shifting with a derailleur. This improved multiple sprocket assembly has special advantages when used with motorized automatic shifting mechanisms.
One object of the present invention is to provide a large sprocket that provides a smooth up shifting action between a large sprocket to a small sprocket.
Another object of the present invention is to provide a sprocket assembly with at least one large sprocket and at least one small sprocket for shifting a chain from the large sprocket to the small sprocket relatively easily and reliably even under a heavy drive load.
The foregoing objects of the present invention can be attained by providing a large sprocket basically having a sprocket body with a first axial side and a second axial side, and a plurality of circumferentially spaced teeth extending radially and outwardly from an outer periphery of the sprocket body. The teeth of the sprocket include a plurality of up shift teeth. The up shift teeth has at least a first up shift tooth, a second up shift tooth located adjacent the first up shift tooth and a third up shift tooth located adjacent the second up shift tooth. The first, second and third up shift teeth are dimensioned to maintain alignment of a bicycle chain to prevent an up shift of the chain when an outer link plate of the bicycle chain meshes with the second up shift tooth, and to permit an up shift the bicycle chain when an inner link plate meshes with the second up shift tooth along a first up shift path.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.